1. Field of the Invention
This invention relates to the field of art of tuning screws used in cavities and waveguides.
2. Prior Art
Known tuning screws provide an impedance adjusting element in the form of a plug, rod or slug which is inserted through an opening in the wall of a waveguide or cavity. The impedance is adjusted by rotating the screw in order to vary the depth of penetration into the waveguide cavity. Prior tuning screws have left much to be desired in that they have suffered from one or more of the problems of lack of repeatability, noisy tuning and insertion loss.
Lack of repeatability results from the change in position of the short circuit between the threads of the tuning screw and the threaded wall when the tuning screw is rotated from a set position and returned to that set position. In view of the inherent variations in thread clearance, the place on the threads of the tuning screw that actually forms the short circuit with the threads in the wall varies and cannot be predicted as the tuning screw is rotated from one position and then returned.
Noisy tuning in prior tuning screws has occurred since the contact between screw and wall has become intermittent as the screw is rotated. Specifically, the short circuit contact between screw and wall threads jumps from place to place upon rotation and such a movable short circuit effectively produces impulse noise.
Ideally, a tuning screw should represent a reactive device with no resistive contribution. A resistive contribution objectionably increases insertion loss in the cavity or waveguide. In prior tuning screws, the short circuit between the threads of screw and wall occurs at some variable distance from the interface of the plug or slug and the actual inner surface of the wall. This extra path length from the interface introduces an objectionable resistive component thereby increasing the insertion loss.
Microwave locking screw assemblies have been known in which there is provided a slot on each side of one of the threads on the screw. This single thread is then deformed to provide an interference when mated with a nut thereby removing any backlash between the male and female threads with slots formed with respect to the split thread. The tuning screw is threaded into an insert and there is thus provided spring loading between the tuning screw and threaded insert to take backlash out of the threads. Such assembly still has left much to be desired in that the points of shorting contact have still varied and not been predictable and vary from thread to thread.